The present invention relates to a printed wiring board formed by adhering a metallic foil such as a copper foil to both sides of a laminate comprising a paper substrate, for example, a paper-phenol material as a base, and to a method of manufacturing the same.
Hitherto, printed wiring boards utilized for wiring in various electronic apparatuses include those ones in which copper foil lands for a plated-through hole provided on both sides are electrically connected in order to enhance the mounting density.
As one of techniques for connecting by a plated-through hole, there is a technique in which a crude hole for plated-through hole is filled with a conductive paste, whereby copper foil lands provided on both sides of the printed wiring board are electrically connected. Among others, those printed wiring boards in which a plated-through hole is formed by using a copper paste obtained by kneading a fine copper powder and a thermosetting resin as the conductive paste are called, for example, copper paste plated-through hole printed wiring boards.
In the case of producing the copper paste plated-through hole printed wiring board as mentioned above, for example, copper foil lands for plated-through hole are provided on both sides of a base substrate to be a base of the printed wiring board, and thereafter a crude hole for the plated-through hole is formed in the copper foil lands. Then, the crude hole is filled with the copper paste, followed by drying and curing to obtain a copper paste plated-through hole.
However, the copper paste plated-through hole printed wiring board as mentioned above has had the problem that the surfaces of the copper foil lands provided on both sides are flat and, accordingly, the adhesion strength between the copper foil lands and the copper paste is weak.
In consideration of the above situation, the present applicant et al. have proposed a printed wiring board in which the adhesion strength between a copper paste and copper foil lands is enhanced such as by forming a hollow portion in the surface of the copper foil lands for plated-through hole (Japanese Patent Laid-open No. Hei 10-206277).
In ordinary printed wiring boards, for example, a paper-phenol substrate, a paper-epoxy substrate, a paper composite substrate and the like can be utilized as a base substrate. However, in copper paste plated-through hole printed wiring boards, the paper-phenol substrate cannot be utilized because of the following production problems.
For example, in production process of a copper paste plated-through hole printed wiring board, as shown in FIG. 15, copper foil lands 101 for plated-through hole are provided on both sides of a base substrate 100, and a crude hole 102 for the plated-through hole is bored in the copper foil lands 101. Then, the crude hole 102 is filled with a copper paste 103, followed by a drying step at about 60 to 100xc2x0 C. and a thermal curing step at 150 to 160xc2x0 C., thereby forming a copper paste plated-through hole.
In such a production process, however, when a paper-phenol substrate is used as the base substrate 100, volatile components contained in the base material resin of the paper-phenol substrate such as methanol, 1-butanol, 2-methyl-1-propanol, formaldehyde, toluene, salicylaldehyde and the like and water contained in the paper material are generated as out gases in the drying step and the thermal curing step.
The out gases generate a burst (blister) 104 shown in FIG. 15 or bubbles 105 shown in FIG. 16 in the copper paste 103 filling the crude hole 102 of the base substrate 100, causing substrate defects.
In consideration of the above situations, the present applicant et al. have studied intensively for preventing the burst 104 and the bubbles 105 which would be generated in the copper paste 103, and have proposed a printed wiring board in which, for example, a through-hole for discharging the gases is provided in the vicinity of a via hole formed by filling with the copper paste 103 (Japanese Patent Laid-open No. Hei 11-177497).
In recent years, attendant on reductions in the cost of various electronic apparatuses, there has been an increasing demand for reduction in the cost of copper paste plated-through hole printed wiring boards.
Therefore, also in the copper paste plated-through hole printed wiring boards, it is desired to form the base substrate from a paper-phenol based substrate material which is available most inexpensively and easily.
In consideration of the above situations, the present applicant et al. have again studied whether or not the copper paste plated-through hole printed wiring board can be formed by use of the paper-phenol substrate 100.
As a result, the present applicant et al. have found that, in the case of producing a copper paste plated-through hole printed wiring board by the previously proposed invention (Japanese Patent Laid-open No. Hei 10-206277), a sufficient adhesion strength has not yet been obtained between the copper foil lands 101 and the copper paste 103 for the following reasons.
FIG. 17 shows an example of structure obtained where a copper paste plated-through hole printed wiring board is formed by use of a paper-phenol substrate as a base substrate.
The copper paste plated-through hole printed wiring board shown in FIG. 17 comprises a base substrate 100 formed of a paper-phenol substrate, and copper foil lands 101 provided on both sides of the base substrate 100. Then, a crude hole 102 for plated-through hole is bored in the copper foil lands 101, and the crude hole 102 is filled with a copper paste 103, followed by drying and curing to produce a copper paste plated-through hole.
In the case of producing the copper plated-through hole printed wiring board in the manner as mentioned above, a destructive test of solder heat resistance and the like after moisture absorption conducted as evaluation of reliability of the wiring board results in that exfoliation 106 is generated between the copper foil lands 101 formed in the periphery of the crude hole 102 and the copper paste 103.
Particularly, when the case where a both side copper-clad laminate with a paper substrate such as a paper-phenol substrate as a base is used as the base substrate 100 was compared with the case where a both side copper-clad laminate with a glass substrate such as a glass-epoxy substrate is used as the base substrate 100, it was found that the exfoliation 106 at the interface between the copper paste 103 and the copper foil lands 101 is generated more easily in the case of the both side copper-clad laminate using the paper substrate as a base.
The exfoliation 106 generated at the interface between the copper paste 103 and the copper foil lands 101 is said to be due to the difference in thermal expansion and contraction characteristics between the both side copper-clad laminate with the paper substrate as a base and the both side copper-clad laminate with the glass substrate as a base.
Here, the thermal expansion and contraction characteristics of the paper-phenol substrate are shown in FIG. 13, whereas the thermal expansion and contraction characteristics of the glass-epoxy substrate are shown in FIG. 14.
The thermal expansion and contraction characteristics shown in FIGS. 13 and 14 are measured by the so-called TMA method in which a specimen is heated up from room temperature at a rate of 10xc2x0 C./min and coefficient of thermal expansion in the thickness direction is measured on a thermal analysis apparatus.
When the thermal expansion and contraction characteristics shown in FIGS. 13 and 14 are compared with each other, it is seen that the paper-phenol substrate shown in FIG. 13 shows a greater size change coefficient in the thickness direction relative to the heating temperature change, as compared with the glass-epoxy substrate shown in FIG. 14.
This means that the glass-epoxy substrate has a lower coefficient of thermal expansion in the thickness direction and a smaller moisture absorption of base material whereas the paper-phenol substrate has a higher coefficient of thermal expansion in the thickness direction and a larger moisture absorption of base material.
As a result, in the case of forming the copper plated-through hole printed wiring board by using the paper-phenol substrate as the base substrate 100, the expansion stress at the time of solder heat resistance is greater as compared with the case of using the glass-epoxy substrate, and exfoliation 106 is generated between the copper foil lands 101 and the copper paste 103.
Particularly, it was found that the exfoliation 106 would easily occur between the copper foil lands 101 in the periphery of the crude hole 102 and the copper paste 103.
Accordingly, in order to produce a copper paste plated-through hole printed wiring board by using a paper-phenol based substrate material which is available most inexpensively and easily, it has been required to further enhance the adhesion strength between the copper foil lands and the copper paste by preventing the exfoliation between the copper foil lands and the copper paste which would occur in the periphery of the crude hole for plated-through hole.
The present invention has been made in consideration of the above-mentioned situations.
According to an aspect of the present invention, there is provided a printed wiring board formed by adhering a metallic foil to both sides of a laminate comprising a paper substrate, wherein metallic foil lands provided on both sides of the paper substrate and provided with a through-hole to be filled with a conductive paste are each provided with a metallic foil removed portion of a predetermined shape in contact with at least a portion of the through-hole.
According to the printed wiring board of the present invention as above-mentioned, since the metallic foil removed portion is provided in the periphery of the through-hole where exfoliation of the metallic foil lands would hitherto occur easily, the conductive paste would not easily be exfoliated from the metallic foil lands, and the adhesion strength between the metallic foil lands and the conductive paste can be enhanced.
In addition, according to another aspect of the present invention, there is provided a method of manufacturing a printed wiring board formed by adhering a metallic foil to both sides of a laminate comprising a paper substrate comprises the steps of: forming metallic foil lands on both sides of a both side metallic foil clad laminate; providing each of the metallic foil lands with a metallic foil removed portion of a predetermined shape; forming a through-hole in the metallic foil lands; and filling the through-hole with a conductive paste and drying and curing the filled conductive paste.
According to the method of manufacturing a printed wiring board of the present invention, the step of forming the metallic foil lands and the step of providing the metallic foil lands with a metallic foil removed portion of a predetermined shape can be carried out simultaneously by only changing a resist pattern, and, therefore, it is possible to provide the metallic foil lands with a metallic foil removed portion of a predetermined shape without increasing the number of manufacturing steps.
The above and other objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings which show by way of example some preferred embodiments of the invention.